One form of method of removing SO.sub.x and NO.sub.x from effluent gas which can be used for example for cleaning flue gases from a power station which is operated with fossile fuels comprises firstly passing the effluent gas through a desulfurisation plant and, after a feed of ammonia, through an SCR catalyst bed for the reduction of at least a part of the nitrogen oxides contained in the effluent gas. Herein SCR means Selective Catalytic Reduction. The general aim at the present time in that flue gases of that kind, when passing into the atmosphere, contain no or only a little SO.sub.x and NO.sub.x.
In order to reduce the level of emission of nitrogen oxides, use is predominantly made of the catalytic reduction of NO.sub.x with ammonia, to give nitrogen. There are a number of methods which can be used for that purpose. In the SCR method, for example, catalysts of different compositions, for example comprising ceramic or ceramic material and/or metal/metal oxide in the form for example of honeycomb bodies and plates are used. The temperatures at which such SCR-catalysts are employed is generally around 300.degree. C.
In one method of removing sulfur and nitrogen oxides from a flue gas, as disclosed in German laid-open application (DE-OS) No. 32 32541, the flue gas, for removal of the sulfur oxides, is passed through a bed consisting of a coal-bearing adsorption agent, and ammonia is added to the gas to remove the nitrogen oxides. In that process, at least two coal-bearing adsorption beds are used, one of which serves as a catalyst in the operation of removing the nitrogen oxides. The other adsorption bed serves for removing from the flue gas the sulfur oxides and ammonia which had not been consumed in the catalyst bed for conversion of the nitrogen oxides.
As indicated above, the present invention is generally concerned with a method of removing sulfur oxides and nitrogen oxides from flue gas, in which an SCR-catalyst bed is used for removal of the nitrogen oxides or at least a part thereof. In comparison with the use of a bed comprising carbon-bearing catalyst material, for example coke, such a method using an SCR-catalyst bed has the advantage of giving a longer operating life, although the level of activity of an SCR-catalyst also experiences a slight reduction, in the passage of time. As will be appreciated, a necessary condition for a longer operational life is that the flue gas which flows through the SCR-catalyst should not include any components which are deposited on the catalyst or which in some other fashion have such a severely adverse effect on the effectiveness of the catalyst that the catalyst has to be replaced or processed after a relatively short period of time.